The present invention relates to a loop closing circuit for detecting a ringing signal, forming a DC loop circuit and sending dial signals (such as dial pulses or multi-frequency signals) in an instrument which is connected to a subscriber's line of an exchange.
Conventional circuits of this type have photo couplers individually provided for an incoming call detecting operation and for an office loop current detecting operation.
What is intended to mean by the "photo coupler" herein mentioned is one that employs a light emitting diode as a light emitting element and a photo transistor, a photo thyristor or the like as a light receiving element. The light emitting diode used as a light emitting element emits light only when a current flows therethrough in a forward direction, and when it is put to use, it is essential to supply a current in the forward direction. In general, the light emitting diode is readily broken by a relatively low reverse voltage, and when flows therein a reverse current, it does not emit light, and hence does not performs the function of a photo coupler. Moreover, conventional circuits is defective in that the number of parts used is relatively large as will be seen from the below description.